Quote:
Originally Posted by bikepilot
Bike engines in general aren't very efficient at all.
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Something to take into account is that maintaing speed with a 800lb loaded motorcycle at 70 mph may require more power than a 3,000lb car at the same speed. I haven't found much data for motorcycle Cd (rider position, custom windshield, highway pegs, etc. make a big difference, for instance), but the limited data I've seen is in the Cd>1.0 range, so a Goldwing probably requires more power to cruise at 70mph than does an Accord, let alone civic.
The Hex head BMWs (R1200 not in cruiser body) seem to be able to get 50 mpg, but I definitely agree on maintenance difficulty and cost, although replacing a clutch in a car application might be more like any other replacement depending on how it's mounted. Wet clutch=good, though. Cooling can be managed, as there are provisions for a thermostatically-controlled oil cooler. A fan to provide airflow over the air-cooled parts when slow or stopped, probably of similar capacity to a radiator fan, should manage that, but water-cooled would be easier, I think.
I have an older BMW, 1997 R1100RT, it has modern engine-control systems, and gets 40-45 mpg while pushing around my oversized windshield, big upper fairing and generally terrible aerodynamics. Bike + me + gadgets = ~900lbs.
I can reduce drag noticeably by lowering the windshield a couple inches. I've done it with a throttle lock engaged, lowered the windshield, pick up 5mph, raise the windshield, lose 5mph on a flat road. I suppose if I did the math, I could figure out something from that data, but I'm too lazy to do it.
It'd be interesting to see BSFC numbers for these motors, but I'm guessing that's not going to happen.